The serial type thermal printer generally includes one printing head and one ink film cassette mounted on a carriage. This construction has only a limited possibility of improvement in terms of function. For example, such a printer permits printing only in the color of an ink film contained in the cassette at a time. Printing in another color requires a change of the cassette. It is therefore conceivable to provide a printing section of composite structure.
The Japanese patent application laid open under No. 59-218887 discloses a printer wherein two ink film cassettes are arranged one above the other and are switchable therebetween to carry out printing during a backward movement as well as a forward movement of the carriage. The Japanese patent application laid open under No. 58-187395 discloses a printer which employs a two-tier ink film with an upper color tier and a lower black tier, the color tier including repeated series of yellow, magenta and cyan. This ink film is vertically shiftable in accordance with color printing and black printing.
The former has the advantage of high speed printing achieved by two-direction printing. The latter has the advantage of enabling color printing and black-only printing without changing the film. However, these printers invariably require a mechanism for vertically shifting the cassettes or films, which results in complicated constructions. Although the former has the advantage of high speed printing over a printer which prints only during a movement of the carriage in one direction, it is necessary to switch between the vertically arranged ink cassettes after printing each line. This switching operation takes time and obstructs achievement of a satisfactory printing speed.